1. Field of the Invention
The present invention generally relates to an apparatus for road path searching, and particularly to an apparatus which can search for a road path in a short time. This invention has particular applicability to a car navigation system.
2. Description of the Background Art
Recently, a navigation system to be carried on a car has been developed and is commercially available. Road path searching apparatuses are provided in some such navigation systems. Upon designation of a destination, the road path searching apparatus detects a road or a path most suitable for getting to the destination from the present location, and displays the path on a CRT display. An example of a conventional road path searching apparatus is described in Japanese Patent Laying-Open No. 62-86500. By employing a road path searching apparatus, a driver can take the displayed most suitable road and arrive at the destination in the shortest time.
A conventional road path searching apparatus, however, requires a long time to detect a road or a path to be taken by a driver. Generally, a plurality of road maps with different detailedness (level of detail) or amount of data for showing one area are previously provided for in a navigation system. When path searching is performed by a road path searching apparatus, a location of the present time and a destination are designated or set in the first place. Generally, as the present location and the destination often face relatively narrow roads, designation or setting thereof is performed based on detailed or high detailed road map data. Thus, when a road path searching apparatus searches for a preferable road path, the search is performed based on the detailed road map data. Generally, there are a great number of roads to be taken on the way from the present location to the destination on a detailed road map. A great amount of processing is required to select the suitable one out of the enormous number of road combinations. As a result, it takes a long time to search for a preferable road path. In addition, even if the road path is detected taking a long time, the detected path often includes narrow roads. In practice, as a driver generally does not like to drive on narrow roads on the way, the detected road or path is not useful for the driver in some cases.
A further example of a conventional road path searching apparatus is described in Japanese Patent Laying-Open No. 61-213718. Also in the disclosed road path searching apparatus, road map data of different detailedness for one area are provided for in advance. In the retrieving operation, after road searching based on a less detailed road map, or road map data of low detailedness, road searching based on road data of high detailedness is performed. That is to say, the detailedness of the road data used in searching operation gradually advances from low level to high level. More specifically, first, main intersections to be passed are detected based on a simple road map, and next, intersections between the detected main intersections are detected based on a road map more detailed by one level. In other words, each portion between one intersection and the other intersection detected on a less detailed road map is interpolated by intersections detected on a map containing an additional level of detail. By repeating the interpolation process from low level to high level of detail, intersections to be passed are determined. As a result, time required for road searching is reduced compared to the above described example of the prior art.